Generally, as an exhaust clarification apparatus provided in an exhaust system of an internal-combustion engine such as a diesel engine, NOx catalyst has been known to clarify NOx (nitrogen oxide) included in exhaust gas. On the other hand, when NOx catalyst has an abnormality such as deterioration or failure, the NOx clarification capacity is deteriorated to thereby cause a situation where NOx is emitted to air in a greater amount than in a normal case. To prevent this, diagnosis of an abnormality of NOx catalyst has been carried out. In particular, in the case of an engine mounted in an automobile, there has been a demand for carrying out a catalyst abnormality diagnosis in an on-board status.
For example, Japanese Patent Application Laid-Open No. H11-93647 (1999) discloses a deterioration diagnosis apparatus for a selective reduction NOx catalyst reducing NOx when reducing agent is supplied. According to this, a catalyst downstream-side NOx concentration when reducing agent is supplied to NOx catalyst and a catalyst downstream-side NOx concentration when reducing agent is not supplied to the NOx catalyst are detected. Based on these NOx concentrations, the catalyst deterioration is diagnosed.
Japanese Patent Application Laid-Open No. 2003-214153 discloses a technique to add reducing agent when NOx storage reduction catalyst reaches a light off determination temperature and a deterioration of the NOx catalyst is determined based on a temperature difference between exhaust gases of the catalyst upstream and the catalyst downstream. Japanese Patent Application Laid-Open No. H7-26944 (1995) discloses a technique to compare a catalyst deterioration determination criterion depending on an engine operation status with a deterioration level calculated based on a catalyst temperature to determine the catalyst deterioration.
By the way, in the apparatus disclosed in Japanese Patent Application Laid-Open No. H11-93647 (1999), there is a problem where emission is deteriorated during a diagnosis of NOx catalyst deterioration. Specifically, this technique intentionally causes a status where no reducing agent is supplied in order to perform a diagnosis of NOx catalyst deterioration. This prevents NOx catalyst from reducing NOx while no reducing agent being supplied, thus causing the emission of NOx.
In view of the above, it is an objective of the present invention to provide an abnormality diagnosis apparatus and an abnormality diagnosis method for NOx catalyst by which the emission during the abnormality diagnosis can be prevented from being deteriorated.